This invention relates to a method for correcting color photographic image data in the case of detecting image data for pixels of a whole frame on an original color film such as a film negative which is generally used in a photographic printer.
It is necessary to photometrically measure the density of an original film such as a film negative in order to determine the printing exposure value or correction value thereof in a photographic printing system. In the prior art, the film negative has its LATD (Large Area Transmittance Density) measured by means of photosensors such as photodiodes provided near an optical path in the photographic printing system. The LATD method, however, is an image detection method used to photometrically measure the film negative generally but does not measure the image density of the film negative correctly or across the whole frame. The printing exposure value or correction value obtained by the LATD method therefore does not quite meet strict requirements. When the film negative image is printed on a photographic paper, it is necessary to change the exposure value or correction value depending on the size, manufacturer or sensitivity of the film negative due to the different in light diffusion particular to each film. These differences are conventionally observed and determined macroscopically and inputted manually from a keyboard or determined by a separate device, and the signals thereof are read out from a data transmission medium. Such key manipulation or signal processing is not only cumbersome but susceptible to input errors.
FIG. 1 shows a system which has been proposed by this applicant to solve the problems encountered in the prior art.
A film negative 2 is conveyed by a conveying mechanism 9 to a position on a film negative carrier 1. The film negative 2 is illuminated by the light from a light source 4 via a color compensation means 3 which comprises 3-primary color filters of yellow (Y), magenta (M) and cyan (C). The light transmitted through the film negative 2 is directed to reach a photographic paper 7 via a lens unit 5 and a black shutter 6. The photographic paper 7 is wound around a supply reel 7A and reeled on a take-up reel 7B in synchronism with the movement and suspension of the film negative 2. Photosensors 8 such as photodiodes are provided near the lens unit 5 of the film negative 2 in order to detect image density data of three primary colors. In accordance with the detection signals from such photosensors 8, picture printing is carried out. An image data detecting apparatus 10 comprising a two-dimensional image sensor 11 is positioned near the film negative 2 at a position inclined from an optical axis LS of the light source 4 and the film negative 2. A lens unit 12 is provided in front of the two-dimensional image sensor 11 to substantially focus the center area of the film negative 2. On the back of the image data detecting apparatus 10 is attached a substrate board 13 for mounting a processing circuit comprising integrated circuits and so on.
The two-dimensional image sensor 11 comprises, as shown in FIG. 2, an image pickup section 101 for optically picking up an image, a storage section 102 for storing charges transmitted from the image pickup section 101, and an output register 103 for outputting the charge stored in the storage section 102. By controlling driving signals 101S through 103S from a driving circuit, the image data in two-dimensions is photoelectrically converted and outputted serially from the output register 103 in the form of an analog image signal PS. The circuit mounted on the substrate board 13 has, for example, a circuit structure as shown in FIG. 3. The image sensor 11 is driven by driving signals 101S through 103S supplied from the driving circuit 20. The light illuminating the image pickup section 101 of the image sensor 11 is outputted from the output register 103 as a picture signal PS, sampled and held by a sample-and-hold circuit 21 at a predetermined sampling cycle. The sampled data is converted by an analog-to-digital (A/D) converter 22 into digital signals DS. The digital signals DS from the A/D converter 22 are inputted to a logarithmic converter 23 for obtaining density signals DN. Then the density signals DN are written in a memory 25 through a write-in control circuit 24 which is controlled by a reading speed signal RS from the driving circuit 20.
A reading speed signal RS from the driving circuit 20 is inputted into the write-in control circuit 24 in order to read out image data at a predetermined speed when the image sensor 11 is driven. The write-in control circuit 24 writes in the density signals DS at predetermined positions of a memory sequentially and correspondingly with the driving speed of the image sensor 11. In other words, the reading speed of the image sensor 11 is determined by the driving speed. The reading speed in turn determines the segmentation number of picture elements with respect to an image area. The memory 25 should therefore store the detected data in correspondence with the number of pixels, too.
When a picture is printed in a conventional manner in the above mentioned structure, the light transmitted through one frame of a film negative 2, which has been conveyed to an standing still at a printing position, is detected by photosensors 8. Then, the filters in the color compensation means 3 are adjusted in response to the picture signals for each of the primary RGB colors (i.e.--red, green, and blue) and the black shutter 6 is opened to expose the photographic paper 7 with the proper exposure value.
An image data detecting apparatus 10 comprising a two-dimensional image sensor 11 of an area scanning type such as a CCD is mounted at a position near the film negative 2 at an inclined angle with respect to an optical axis LS to facilitate the mounting thereof. The whole frame of the film negative 2 is segmented into a large number of arrayed pixels for detecting image data. In other words, when predetermined driving signals 101S through 103S are fed from the driving circuit 20 to the image sensor 11, the two dimensional image sensor 11 is adapted to receive the light transmitted through the film negative 2 on the printing section via the lens unit 12. The two-dimensional image sensor 11 can therefore scan the whole surface of a frame of the film negative 2 along the scanning lines SL sequentially by segmenting the whole area into a large number of small arrayed pixels 2A as shown in FIG. 4A. In accordance with the scanning of the whole area, the output register 103 of the image sensor 11 outputs picture signals PS sequentially, then the picture signals PS are sampled and held by a sample-and-hold circuit 21 and the sampled values thereof are converted by an A/D converter 22 into digital signals DS. The digital signals DS from the A/D converter 22 are logarithmically converted by a logarithmic converter 23 to density signals DN. The density signals DN are controlled by a write-in control circuit 24 to be stored in a memory in the arrays corresponding to the pixels 2A shown in FIG. 4B and in terms of the density digital values of the negative film 2.
If the digital values for respective pixels of the film negative 2 or the density values for respective pixels with respect to three primary colors are stored in the memory 25, it is possible to read out the digital values for any particular pixel of the film negative 2 from the memory 25. If the density values for respective three primary colors of R, G and B when using mosaic filters (not shown) are stored as shown in FIG. 4B, it is possible to read out such values from the memory for processing (which will be described hereinafter) in order to determine the exposure or correction values for photographic printing in the same manner as in the prior art.
The image sensor 10 consists of a predetermined number of elements, and in accordance with the number of elements, the number of divided elements of the whole surface of the image are physically decided. In the case where the image sensor 10 is moved relatively with respect to the film negative 2, however, the number of elements may be changed by changing the read-out speed.
The image data, as shown in FIG. 4B, detected and stored by the image sensor 25 in the manner described above, does not exhibit correct data of the image itself of the film negative 2 due to the influences of an unevenness of the elements constituting the image sensor 10, a shading of the light source 4, an aberration of the lens unit 11 and the difference in the base density of the film negative 2. For the reasons described above, in the case where it is necessary to use the stored data from the memory 25 as is, the influence of the unevenness of the constituting elements is directly felt, and accordingly, correct image data cannot be obtained. In addition, in case of an original color film, it is necessary to store in the memory the image data regarding the three respective primary colors R, G and B. However, in the prior art method, color unbalance may be caused by the local fading of the filters with respect to the primary colors, and moreover, color temperature variations and base color differences of the film negative 2 may be observed due to chronological changing of the light source. These adverse phenomena may result in the loss of the white balance.